As modern electronic circuit boards evolve toward increased circuit and component densities, thorough board cleaning after soldering becomes more important. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with flux, and thereafter passing the flux-coated board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes solder adhesion. Commonly used solder fluxes generally consist of rosin, either used alone or with activating additives, such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the remaining flux and its residues are often removed from the circuit boards with an organic solvent. The requirements for such solvents are very stringent. Defluxing solvents should have the following characteristics: be low boiling and nonflammable while having low toxicity and high solvency power, so that flux and flux residues can be removed without damaging the substrate being cleaned.
While boiling point, flammability and solvency characteristics can often be adjusted by mixing different solvents together, the mixtures that are formed are often unsatisfactory because they fractionate to an undesirable degree during use. Such solvent mixtures also fractionate during distillation which makes it virtually impossible to recover a solvent mixture with the original composition.
On the other hand, azeotrope-like mixtures, with their constant boiling point and constant composition characteristics, have been found to be very useful for these applications. Azeotrope-like mixtures exhibit constant boiling points and, for all practical purposes, they do not fractionate on boiling. These characteristics are also important when using solvent compositions to remove solder fluxes and flux residues from printed circuit boards. Preferential evaporation of the more volatile solvent mixture components would occur if the mixtures were not azeotrope-like. This would result in the mixtures having changed compositions and possibly less desirable solvency properties, such as lower rosin flux solvency and lower inertness toward the electrical components being cleaned. This azeotrope-like character is also desirable in vapor degreasing operations, where redistilled solvent is generally employed for final rinse cleaning.
In summary, a vapor defluxing or degreasing system operates as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is azeotrope-like, fractionation will occur, and undesirable solvent distributions will result which could detrimentally affect the safety and efficacy of the cleaning operation.
A number of chlorofluorocarbon-based azeotrope or azeotrope-like compositions have been discovered and, in some cases, used as solvents for solder flux and flux residue removal from printed circuit boards, and also for miscellaneous degreasing applications. Some of the chlorofluorocarbons which are currently used for cleaning and other applications have been theoretically linked to depletion of the earth's ozone layer. As early as the mid-1970's, it was known that introduction of hydrogen into the chemical structure of the previously fully-halogenated chlorofluorocarbons reduces the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade more rapidly in the lower atmosphere and not reach the stratospheric ozone layer to any appreciable degree. What are needed, therefore, are substitute chlorofluorocarbons which have low theoretical ozone-depletion potential.
The chlorofluorocarbons of the constant boiling, azeotrope-like mixtures described herein contain hydrogen in their chemical structures. These chlorofluorocarbons have very low ozone-depletion potentials and are expected to decompose almost completely prior to reaching the stratosphere.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes or constant boiling, azeotrope-like mixtures. This fact obviously complicates the search for new constant boiling compositions which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like compositions which have desirable solvency characteristics and, particularly, greater versatility in solvency power.